Mrs. Blass is tired of her students not being prepared for class each day. She knows that they are hard-working seniors who value getting an A in the class, and she wants to set up a schedule of work so they study hard and consistently. She schedules pop quizzes. Only she knows when those quizzes will be given. Assuming the students find the grade of A or B to be reinforcing, what schedule of reinforcement is at work here?